Heterogeneous networks are an efficient network deployment solution for satisfying the ever-increasing demand of mobile broadband services. In a heterogeneous network, a low- or lower-power node (LPN), for example a picocell, microcell or femtocell base station, is placed in a traffic hot spot within the coverage area of a high- or higher-power node, for example a macrocell base station, to better serve nearby mobile devices. Deploying a low power node in a traffic hot spot may significantly reduce the load in the macro or other higher-power cell covering the area.
The traffic uptake of an LPN however may be somewhat limited. This is due to the transmit power difference between a macro base station (BS) and an LPN. This is illustrated in FIG. 1. In FIG. 1, the network 2 is shown as comprising a low power node 4 (e.g. picocell or microcell base station) placed within the coverage area of a macrocell base station 6. Each of the LPN 4 and macrocell base station 6 are connected to a radio network controller (RNC) 8, which in turn connects to a core network 10. As downlink signal strength or quality is used as a basis for triggering a hand over between base stations, the border 12 of the LPN cell is determined by the downlink (DL) signal strength or quality as measured by a mobile communications device 12 (also known as a user equipment—UE). The border 12 occurs at the point where the downlink signal strength or quality from the LPN 4 is the same as the downlink signal strength or quality from the macrocell base station 6. As the LPN 4 has a much lower transmit power level compared to the macrocell base station 6, the cell border 12 is much closer to the LPN 4 than the macrocell base station 6. However from the uplink (UL) perspective, the base station transmit power difference is irrelevant and the UE 14 would be best served by the base station to which it has the lowest path loss. Thus, for the UL, the cell border should be somewhere near the equal-distance point between the two base stations 4, 6 since at the equal-distance points the path loss from the UE 14 to both base stations 4, 6 is approximately equal.
The region 16 between the UL ‘border’ and DL border 12 is often referred to as the imbalanced region 16. In the imbalanced region, the UL from the UE 14 would generally be better served by the low-power node 4 (as it is closer to the UE 14 than the macrocell base station 6), but the DL would be better served by the macrocell base station 6. However, as cell selection is determined on the basis of the DL signal quality, a UE 14 in the imbalanced region 16 will generally be served by the macrocell base station 6, meaning that the UE 14 cannot take advantage of the better UL to the LPN 4.
A further problem with UEs 14 in the imbalanced region 16 is that UL transmissions by a UE 14 to the macrocell base station 6 can cause interference at the LPN 4 (particularly where the UE 14 is close to the DL border 12).
In addition, conventional handover decisions based on DL signal quality may result in the traffic-uptake of the LPN 4 being less than desired by the network operator.
A number of techniques and solutions are available for UEs 14 that can be used to improve the communications performance of UEs 14 that are located in the imbalanced region 16 of a network 2. For example, the range of the LPN 4 can be effectively extended by a UE 14 applying an offset to the measurements of downlink signal quality from the LPN 4, and using the adjusted downlink signal quality measurement in cell selection decisions (which means that the UE 14 can be served by the LPN 4 while it is in at least a portion of the imbalanced region 16). In another example, a form of soft handover can be used in which the macrocell base station 6 remains as the serving cell for the UE 14 when it is in the imbalanced region 16, but the UE 14 establishes a data connection with both the macrocell base station 6 and the LPN 4. Other techniques and solutions will be apparent to those skilled in the art.
Although many of these techniques or solutions are successful in improving the performance of the UE 14 in the imbalanced region 16, they require actions by the nodes 4, 6, 8 in the network 2 and/or additional signalling to be established with the UE 14, which consumes resources in the network 2. Thus, these techniques and solutions should only be initiated and used when they are needed or likely to be beneficial to the UE 14 (i.e. when the UE 14 is within the imbalanced region 16).